Nanostructured TiO2 dye-sensitized solar cells (DSSCs) have been intensively investigated in the past decade as a promising renewable energy source. In such DSSCs, the dye sensitizer is one of the key components for high solar-to-electrical energy conversion efficiency. Thus, considerable effort has been made in developing photovoltaic sensitizers with high power conversion efficiencies. Until now, simple heteroleptic Ru(II) complexes with anchoring groups have achieved power conversion efficiencies over 11% in standard AM 1.5 sunlight. These efficiencies exceed those of other competitive “low cost-medium efficiency” technologies such as thin film amorphous silicon, cadmium telluride, copper indium gallium selenide, etc., but as yet cannot compete with the standard high efficiency polycrystalline or monocrystalline silicon cells (15-22%) or indeed the new high efficiency compound semi-conductor technologies based upon gallium arsenide (>30%). Besides Ru complexes, metal free organic dyes have also been utilised in DSSCs, and have thus far achieved power conversion efficiencies of over 9% under AM 1.5 G irradiation.
These simple structured Ru(II) complexes and organic small molecules are, however, not fully satisfactory for commercial outdoor device applications because, although high power conversion efficiencies can be observed initially, devices comprising such compounds exhibit poor device stability under long-term light exposure, volatile liquid electrolyte evaporation or leaking and thermal stress. For practical outdoor device applications, long-term device stability at high temperatures (e.g. ca. 80° C.) is an essential requirement in addition to high photoelectric conversion efficiency. There is therefore a continuing need to provide high performance DSSC sensitizers which not only provide high conversion efficiency but also facilitate long-term device stability.